Alchemy JSON RPC Update according to Ethereum Spec

[charlescrain]

Ethereum is deprecating totalDifficult and we should prepare accordingly.

The change would be here:
https://github.com/airalab/hs-web3/blob/master/packages/ethereum/src/Network/Ethereum/Api/Types.hs#L282

for backwards compatibility, we can just make a Maybe:

, blockTotalDifficulty  :: !(Maybe Quantity)

[charlescrain]

Here's the pull request: #142

[akru]

Cool, merged!